The present invention relates generally to amorphous memory devices and more specifically to a method producing amorphous memory device with a substantially low first fire threshold voltage.
Amorphous memory devices, for example, a tellurium based chalcogenide glass are well known. Deposition conditions can be adjusted such that the materials can initially be deposited either in a crystalline state or an amorphous state. Although not incompatible, the deposition of these films has been difficult to integrate with normal silicon process techniques. Similarly, the resulting device requires support circuitry of a higher capability than that of the normal silicon circuits.
To deposit an amorphous film in a crystalline state, the deposition is performed at or above the crystallization temperature resulting in a crystallized film with a conductivity of 10 to 100 (ohm-centimeter).sup.-1 for a typical chalcogenide material. In order to erase these devices from a low resistance crystallize state to a higher resistance amorphous state, high current densities are required, generally in the range of 5.times.10.sup.5 to 1.times.10.sup.6 amperes per square centimeters. Although not impossible, the standard silicon circuitry does not generally provide such high current density capacities, (thus requiring the design of special capacity circuit elements).
When the amorphous film is deposited in the amorphous state, the first fire threshold voltage is typically 2 to 5 times the normal operating threshold voltage. Thus the silicon support circuitry must be capable of supplying the higher voltage levels necessary for the first firing.
In order to reduce the current density and high voltage requirements of amorphous memory devices as first formed, the prior art has attempted to form the memory devices in a condition between completely crystallized state and a completely amorphous state. One approach to depositing partially crystallized material involves adjusting the deposition rate of the material to produce substrate temperatures near the crystallization temperature of the material. Since these films, which are not oxygen contaminated by leaky vacuum systems and other causes, have a sharp crystallization temperature, this approach requires very critical substrate temperature control within the vacuum deposition system. In a production environment, this degree of control is difficult to achieve and economically infeasible.
Another technique of the prior art to lower the first firing threshold voltage of the device is to fire the device at an elevated temperature. Since the first fire threshold voltage decreases only typically 0.03 percent to 0.06 percent per degree centigrade depending upon the material, a sufficient reduction in threshold voltage cannot be achieved in a reasonable operating temperature range.
Although the prior art has attempted to lower the first fire threshold voltage, they have not been able to do so in a repeatable manner.